NEW ZEALAND: Questions about Christchurch earthquake

by Peter Westmore

News Weekly, March 19, 2011

The terrible earthquake which destroyed part of Christchurch's central business district, with the loss of over 200 lives, highlights deficiencies in Christchurch's building code which permitted some relatively new buildings to be destroyed, while many older buildings survived.

The Christchurch City Council's policy with regards to earthquakes, called Earthquake-Prone, Dangerous and Insanitary Buildings, was adopted in 2004 and revised in 2010, before last September's earthquake in Christchurch. Despite the magnitude 7 quake then, it had not been revised prior to last month's events.

The policy recognised that Christchurch was earthquake-prone. It said, "Christchurch City lies in an intermediate seismicity zone some distance from a zone of high activity associated with the Alpine Fault. However, known earthquake sources - in particular the Ashley, Springbank and Pegasus and the new 'Darfield' fault zone - exist within the region and are large and close enough to Christchurch to cause significant damage throughout the city."

It defined "earthquake-prone buildings" as those which had been constructed prior to 1976, an absurdly unrealistic assumption. It said, "Buildings constructed after [1976] are unlikely to be earthquake-prone, although it is recognised that some buildings constructed after 1976 will be, or could become, earthquake-prone."

There is no direct reference in the document to the earth's instability, when it is well known that parts of Christchurch were built on reclaimed wetlands which are prone to liquefaction, a process which involves the rising water-table liquefying the top soil during an earthquake.

It is significant that some of the buildings where there was large loss of life, including the seven-storey CTV building, were built after 1976, while neighbouring buildings, in some cases many years older, were unscathed. The CTV building was built in the mid-1980s.

Over 100 people are believed to have died in the CTV building. Of them, about 15 worked for Canterbury TV; and up to 90 were staff and students from the language school, King's Education. This building was declared structurally sound by council engineers after the September earthquake.

There were also deaths in the PGC Building, a five-storey concrete building erected in the 1960s, which collapsed like a house of cards.

The largest building in Christchurch, the 26-storey Hotel Grand Chancellor, was damaged in the earthquake but did not collapse. Its damage appeared to have been to the foundations, as the building rotated on its base and tilted.

Other nearby tall buildings, including the 76-metre Price Waterhouse Cooper building and the 73-metre Pacific Tower, were apparently undamaged. The earthquake inquiry will have to examine why these were unaffected, while much smaller buildings collapsed.

While attention has focussed on the quarter of the buildings in the central business district, which either collapsed or were so severely damaged that they cannot be repaired, the fact remains that three-quarters of the buildings survived intact. In fact, photos of the flattened CTV building show it surrounded by intact buildings.

The council policy document recognised there were potentially 7,600 buildings in Christchurch which were deemed to be earthquake-prone. Many of these were old brick and masonry structures which simply collapsed due to the vigorous shaking of the ground during the earthquake.

The city council's policy recommended that these be reinforced so as to withstand a "moderate earthquake", but their owners were given between 15 and 30 years to strengthen them.

The code required such buildings to be upgraded to withstand an earthquake "that is one-third as strong as the earthquake shaking ... that would be used to design a new building at that site". Again, this is absurdly unrealistic, and points to the dangerous inadequacy of the building code.

Further, building-owners were given no incentives, as in reductions in council rates or tax breaks, to accelerate building works.

In other earthquake-prone regions, including California, Japan, Taiwan and Chile, there have been long-established building protocols to minimise building damage and loss of life in catastrophic earthquakes.

Among the methods widely used overseas on multi-story buildings are bolted steel frame and box beam structures, the use of pre-stressed rather than reinforced concrete, and deep pile foundations to anchor the floor to underlying bedrock.

Despite government and media claims that Christchurch had one of the most stringent earthquake standards in the world, it seems that more attention was given to protection of "natural assets and habitats" than to earthquake protection. Claims that existing building standards were adequate are nothing more than complacent propaganda.